When starting ezplatform:reindex command with content-count % iteration-count === 0 the command will restart (internally) on reaching 100%.

https://github.com/ezsystems/ezpublish-kernel/blob/6.7/eZ/Bundle/EzPublishCoreBundle/Command/ReindexCommand.php#L366-L380

Changing yield $contentIds to check if empty fixes the issue.

if (!empty($contentIds)) {
    yield $contentIds;
}